1. Field of the Invention
This invention relates to a gasket seal and more particularly, to a gasket seal for use in ultra-high vacuum devices.
2. Description of the Prior Art
Ultra-high vacuum environments, and devices capable of maintaining an ultra-high vacuum are essential in a number of scientific and commercial settings including surface science studies, space simulation chambers and high energy physics research.
For work at pressures less than 10.sup.-8 torr, it is necessary to bake the entire chamber of an ultra-high vacuum device and all its flanges to several hundred degrees Celsius in order to outgas all of the internal surfaces. Vacuum systems of this type thus require near perfect seals at the contact points of the flanges which are able to withstand the temperature and pressure demands of the system and are able to maintain the seals over a wide range of temperatures and pressures.
A standard flange geometry has been developed under the name CONFLAT. This flange geometry has been adopted as an international industry standard for flanges, and requires a soft-metal gasket to seal the CONFLAT joints, which may be manufactured of copper or aluminum. Examples of such gaskets are described in U.S. Pat. Nos. 4,616,860 and 3,211,478.
Although standard for general use, the CONFLAT system usually requires customization for particular uses. For instance, in order to provide inlets for electricity, refrigerants, motion, gas or light into a vacuum system, hermetically sealed feedthroughs must be used. This is presently accomplished by welding or brazing feedthroughs to a flange or to the wall of a vacuum chamber. If it is desired that a feedthrough be demountable it is usually permanently mounted to a demountable flange. The flange is then sealed to a mating flange on the vacuum chamber using a captured elastomeric O-ring or soft-metal ring gasket.
Although popular because of its ability to maintain seal integrity during high temperature cycling, the use of CONFLATS is not without problems. There are four principal problems associated with the insertion and removal of CONFLAT gaskets. First, the gaskets must be handled with polyethylene or latex gloves to avoid fingerprint contamination of that part of their surfaces which will be inside the vacuum system. The use of such gloves is often awkward and expensive.
Second, frequently the orientation of flanges on the vacuum system makes installation of CONFLAT gaskets difficult. Specifically, gaskets have a tendency to fall out of position (often onto dirty floors) during the installation operation.
Thirdly, many times gaskets will fall out of position during the installation, but will remain between the flanges and out of position for sealing, thus leading to mysterious and difficult to locate leaks.
Finally, quite often during removal of CONFLAT gaskets, it is found that the gasket material has flowed to such an extent that the gasket becomes lodged into one of the CONFLAT joint flanges. The removal of such a lodged gasket is difficult as there is less than 0.040 in. of gasket above the uppermost surface of the sealing face. To use sharp implements to dislodge the gasket is often the only means of gasket removal. Such sharp implements can damage the critical sealing surface of the CONFLAT flange, resulting in leakage.
The fabrication of nearly all structures to be used within ultra-high vacuum chambers centers on the CONFLAT flange. This is especially the case if the structure is to be easily removed from the vacuum environment. There are at present no standard mounting fixtures available to the ultra-high vacuum chamber user. CONFLAT flanges are purchased either blank for later machining by the user, or with a custom-made structure. If one wishes to mount a fixture within the vacuum chamber, it is necessary to provide mounting means by machining the inside surface of the CONFLAT flange. These flanges are made of stainless steel, which is difficult to machine, and the machining process exposes the flanges to environments which are harsh for ultra-high vacuum components. The sealing knife edges of the CONFLAT flange are exposed to chips and sharp tooling, and the entire flange is exposed to oil and dirt from machines and machinist's hands. Unless a high degree of attention is paid to cleanliness during machining, the flange must be thoroughly cleaned by a lengthy and costly process following the machining of a flange in a machine shop.
By providing holes in the CONFLAT gasket which receive supporting structure, such as mounting fixtures, the need for machining the CONFLAT flange may be eliminated in many cases. This is especially important to the user if the structure is to be used only temporarily.